Research in reproductive biology of the female has overshadowed that in the male. Application of basic and clinical studies to solving problems in human reproduction has been more frequent and more successful in females than in males. Work to be described in this application is designed to increase our basic and clinical knowledge of male reproduction. The primary goals of this work are the study of infertility and contraception in men and the exploration of the basic biology of the male reproductive system important in solving these clinical problems. The studies included range from the molecular biology and biochemistry of the brain, pituitary and testis through the physiology of the endocrine control mechanisms for testicular function in adults to clinical studies on the effects of varicoceles on fertility. The efforts of investigators with varied backgrounds and capabilities are directed toward solving major problems of male reproductive biology.